We have investigated whether primary monolayer cultures of adult rat hepatocytes may provide a useful model for the study of the mechanism and control of protein degradation. When perfused liver or freshly isolated hepatocytes are used, studies are limited to a few hours duration whereas cultured cells may be kept for several days. Culturing allows for a period of recovery from the trauma of isolation, yields a uniform population of hormonally-responsive cells, and eliminates non-viable cells during the plating process. Cultured hepatocytes are in a dynamic state and degrade protein at a rate similar to that observed in vivo. All proteins appear to be involved as similar rank-order effects are noted whether the period of observation is 3, 6 or 18 hours. The rate of degradation is nutritionally and hormonally sensitive in a consistent and reproducible fashion. On a minimal salts medium, the fractional decay rate (kd) was 0.055 plus or minus 0.005 per hour whereas when a complete medium was used, kd equals 0.035 plus or minus 0.005. About 20 percent of the response can be attributed to the vitamin complement of the complete medium. Most of the effect, however, may be attributed to the amino acid component. The addition of insulin (13 mUnits/ml) lowers the rate of degradation about 0.004/hour regardless of the medium used, thus the effect is an additive one. It appears that this will be a useful model for the study of protein degradation in vitro.